The present invention relates to improved pumping apparatus and methods, and, in particular although not solely, to pumps (centrifugal) to operate in low suction head conditions.
Pumps such as centrifugal pumps have been manufactured for many years. They have been used in boiler feed, mine drainage and petroleum/chemical industry for the pumping of different fluids.
There are many places in process industries where low NPSH (Net Positive Suction Heads) are required for pumping liquids which are at or near their boiling points. In conventional pumps as the NPSH is reduced, cavitation sets in and it eventually becomes extensive enough to cause blockage of the blade passages, efficiency breakdown, and possibly also a gradual destruction of the components of the pump. As is well know, cavitation may be expected in a flowing liquid when the local pressure falls to the vapour pressure of the liquid. Local vaporisation of the liquid will then result, causing development of a vapour filled bubble or cavity in the flow. The low pressure cavity is swept down stream into a region of higher pressure where it collapses suddenly, the surrounding liquid rushing in to fill the void. At the point of disappearance of the cavity the in rushing liquids come together momentarily raising the local pressure within the liquid to a very high value. If such cavity collapse is near the boundary wall, such high local pressure can cause erosion of the wall material. An advantage of pumps having a low NPSH is the ability to reduce holding tank heights (and save construction cost), or to pump fluids which are near their boiling point, or by running the pump faster and replacing multi-stage (booster) pumps.
Development on pumps to reduce or eliminate cavitation at inlet include work on what is known as the super cavitating type of pumps which use an inducer upstream from the pumps impeller. Such an inducer is normally driven by the axle of the impeller and hence increase the demand for power from the motor driving the pump. In general, anti-cavitation pumps can increase the inlet type number (specific speed) of a high speed pump by two to three times, thus giving a generous margin of safety in respect of permissible NPSH.
The provision of a booster pump upstream from the centrifugal pump where cavitation may result is also a common way of overcoming cavitation. The boost pump increases the head of fluid at the centrifugal pump to there by reduce the occurrence of cavitation.
Yet an alternative development to conventional pumps includes the provision of guide vanes upstream of the impeller of the pump wherein the guide vanes provide a swirling motion to the intake fluid to improve the angle of incidence of the fluid flow with the leading edge of the impeller. The problem with both super cavitating pump and pumps having up stream guide vanes is related to the increase in surface area over which the intake fluid is required to flow hence creating a head loss as a result of friction. The booster pump or super-cavitating pump also require a further power input.
A further alternative way in which the performance of a pump may be improved by avoiding cavitation inside of the pump itself. Such cavitation may occur under partial load conditions. In UK Patent Application GB2058218 there is described a means of drawing fluid form the volute casing of a pump, collecting it in a cylindrical reservoir and directing the fluid via openings to the impeller blades at inlet. The configuration of this method is such to induce a backflow within impeller blades which counteracts the occurrence and development of the secondary flows in such a way that cavitation loading of the impeller for part-load conditions is eliminated or reduced. This specification however deals with minimisation of the energy loss as a result of secondary flows, sometimes called re-circulating flows. These are usually in the form of vortices with a direction of rotation opposite to that of the main stream. These are usually found at the tips of the impeller vanes on the exit and inlet sides. The losses are normally accounted for as part of the total loss assumed within the major elements of the pump, which also include frictional losses.
The function of the injection of fluid of the invention of GB2058218 is analogous to providing a series of fixed guide vanes before the impeller in order to guide the fluid in a certain direction in respect of partial load vortices.
U.S. Pat. No. 4,492,516 deals with the losses as a result of impeller re-circulation by the provision of passage ways shaped to direct the flow of fluid to be reinjected at the inlet of the impeller. As such head losses as a result of secondary flows or friction are small compared to the total headloss of the energy equation of fluid flow its reduction or elimination, although having some effect, proportionally it has very little effect on such energy.
It is therefore an object of the present invention to provide improved pumping apparatus and methods, and in particular although not solely to centrifugal pumps to operate in low suction head conditions, or which will at least provide the public with a useful choice.
Accordingly in a first aspect the present invention consists in a pumping system comprising
means defining a pump having an inlet and an outlet,
an inlet conduit connected to said inlet of said pump for the inlet flow of fluid to said pump
a delivery conduit connected to said outlet of said pump for the outlet flow of fluid from said pump
means adapted to bleed at least part of the outlet flow,
means capable of increasing the velocity head of bleed fluid flow, and
means to inject a flow responsive to the condition of bleed fluid flow into the inlet conduit whereby
in operation the injected flow increases at least the velocity head of inlet flow of fluid to said pump.
Preferably said means adapted to bleed is connected for fluid communication by at least one conduit to said means to inject.
Preferably said means to inject is at least one nozzle in fluid connection via said at least one conduit with said means adapted to bleed.
Preferably said fluid connection with said means adapted to bleed and means to inject is controlled by at least one valve.
Preferably one control-valved conduit for fluid communication to each of said at least one nozzle, provides said fluid connection with said means to bleed.
Preferably said means capable of increasing velocity head is in the bleed fluid flow path between said delivery conduit and the said inlet conduit, and is adapted for the flow path for bleed fluid to be reduced in cross-sectional flow area to thereby increase the velocity pressure head of said bleed fluid flow prior to being injected into said inlet conduit.
Preferably said means capable of increasing velocity head is provided at said at least one nozzle in the form of a reduced cross-sectional flow area for bleed fluid flow.
Preferably said at least one nozzle comprises a passage for fluid with and inlet for bleed fluid to be received from said means adapted to bleed (via said conduit) and an outlet connected for fluid delivery to said inlet conduit, wherein said inlet of said nozzle is of greater cross sectional area that said outlet of said nozzle.
Preferably said passage of said nozzle is gradually tapered over the length of said passage between said nozzle inlet and said nozzle outlet such that the cross sectional flow area of said outlet is smaller that the cross sectional flow area of said inlet of said nozzle.
Preferably the ratio of said cross sectional flow area of said nozzle inlet to the cross sectional area of said to said nozzle outlet is substantially four.
Preferably said passage of said at least one nozzle is substantially circular in cross sectional area and the ratio of inlet diameter to outlet diameter is substantially two.
Preferably the ratio of the length of said passage to the diameter of said nozzle inlet is substantially two.
Preferably said at least one nozzle is engaged with said inlet conduit to, in use inject bleed fluid into the inlet conduit through said nozzle outlet at substantially right angles to the main flow direction of fluid in said inlet conduit.
Preferably said at least one nozzle is engaged with said inlet conduit to, in use inject bleed fluid into the inlet conduit through said nozzle outlet to induce a rotational flow to inlet flow of fluid approaching said pump.
Preferably there are at least two nozzles for injecting fluid into said inlet conduit.
Preferably there are four nozzles for injecting fluid into said inlet conduit.
Preferably said at least two nozzles are each positioned the same distance upstream of said pump.
Preferably said at least two nozzles are equi-spaced about the perimeter of said inlet conduit.
Preferably said pump is a centrifugal pump.
Preferably said at least one nozzle is engaged with said inlet conduit to in use inject bleed fluid into the inlet conduit through said nozzle outlet to induce a rotational flow to inlet flow of fluid approaching said pump co-rotatory with the rotational direction of the pump impeller.
In a further aspect the present invention consists in a method of pumping a liquid using a pump of a kind having an inlet conduit for input flow of fluid to the in flow chamber of said pump and an outlet conduit for output flow of fluid from the volute chamber of said pump, said method comprising;
bleeding of fluid from the outlet conduit, of a higher total pressure compared to the input flow total pressure
increasing the velocity energy of the bled fluid and
injecting of the bled fluid into the inlet conduit to increase the velocity energy of said input flow.
Preferably said velocity energy of said bled fluid from said outlet conduit is increased as a result of restricting the bleed flow path area of said bled fluid prior to injecting.
Preferably said bleeding of fluid is a controlled portion of the total output flow.
Preferably said bleeding of fluid is controlled by a control valve in a bleed flow Preferably said bleed flow path is restricted as a result of tapering in the bleed flow conduit between said bleeding and said injecting.
Preferably said control valve is responsive to cavitation and/or performance conditions of the pump whether by means of micro processor, computer or other logic control means responsive to sensors.
Preferably flow or pressure or flow difference or pressure difference activated valving systems without logic control can be used.
Preferably the pump is a centrifugal pump.
Alternatively the pump is an axial flow pump.
Yet alternatively the pump is a mixed flow/diagonal type pump.
Preferably said injecting of bled fluid into said inlet conduit imparts a rotatory flow in the input flow to the pump which is co- rotatory to the rotation of the pump impeller.
Alternatively said injecting of bled fluid into said inlet conduit imparts a rotatory flow in the input flow to the pump which is counter- rotatory to the rotation of the pump impeller.
In still a further aspect the present invention consists in a nozzle unit for injecting fluid into the main inlet flow stream of a pump comprising;
a conduit section capable of insertion into the main inlet conduit defining the flow boundary to said inlet flow stream of said pump
said conduit section comprising
an inlet
outlet
a wall defining region there between to provide a substantial continuation of the boundary to the main inlet flow stream of fluid, and
at least one aperture through said wall defining region
an injection nozzle for said at least one aperture to in use deliver through said aperture a fluid, said injection nozzle comprising;
a conduit having an inlet and an outlet said outlet being at said at least one aperture through said wall defining region such that a fluid communication can be established between the inlet to said conduit of said nozzle and said main inlet flow stream wherein
the flow area of said inlet to said conduit is larger than said outlet to said conduit.
Preferably said conduit of said nozzle is secured to said conduit section such that in use the flow path of injected fluid at said outlet to said conduit is at substantially right angles to the main inlet flow stream of a said pump.
Preferably at least said inlet an outlet of said conduit section are of complementary shape to the boundary defining region of said main inlet conduit
Preferably said wall defining region of said conduit section is defined by a substantially constant diameter bore corresponding to the diameter of said main inlet conduit.
Preferably said conduit of said nozzle is secured to said conduit section such that in use the flow path of injected fluid at said outlet to said conduit is at substantially right angles to the main inlet flow stream of a said pump and at a tangent to said boundary of said main inlet conduit.
Preferably said inlet of the conduit of said nozzle is of a flow area substantially four times that of the flow area of the outlet to said conduit.
Preferably said conduit of said nozzle is in cross section substantially circular.
Preferably said conduit is gradually tapered between said inlet and said outlet over a distance substantially twice that of the diameter of said inlet to said conduit.
This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.
The invention consists in the foregoing and also envisages constructions of which the following gives examples.
Preferred forms of the present invention will now be described with reference to the accompanying drawings in which;